1. Field of the Invention
The invention relates to multicomponent silicone compositions which can be crosslinked via the click reaction and which, after mixing of the individual components, harden to give a silicon polymer which is preferably an elastomeric material.
2. Background Art
Platinum-catalyzed hydrosilylation, in particular, and also tin-catalyzed condensation, are crosslinking reactions which have become widely used in the sector of multicomponent silicone compositions crosslinking at room temperature (RTV-2 silicone rubbers). Both types of reaction permit formation of a stable three-dimensional network via linkage of polyorganosiloxanes using suitable crosslinking agents. The systems known as “addition-crosslinking RTV-2 systems” are based on the (mostly platinum-catalyzed) reaction of alkenyl-functional polyorganosiloxanes with SiH-functional oligosiloxanes. The systems known as “condensation-crosslinking RTV-2 systems” are based on the linkage of Si—OH-functional polysiloxanes via polyfunctional silanes having hydrolyzable groups, for example tetraethoxysilane or tetrapropoxysilane or condensates thereof, proceeding in the presence of water, mostly with tin catalysis. Both crosslinking mechanisms permit simple production of vulcanizates having defined network structures, based on the reactions which proceed almost quantitatively under standard conditions. The resultant elastomeric materials feature specific properties. Examples that may be mentioned of these are: high thermooxidative stability, good low-temperature flexibility, and chemical inertness.
Alongside all of these advantages, these systems also have disadvantages. In contrast with addition-crosslinking systems, condensation-crosslinking systems form a network by eliminating low-molecular-weight units, mostly short-chain alcohols, such as methanol or ethanol. The result of diffusion of these substances out of the vulcanizate is not only problematic relating to health and safety, but also results in volume reduction (“shrinkage”) of the molding. Furthermore, the tin catalysts used, mostly diorganotin(IV) dicarboxylates, create health- and environment-related risks which are difficult to evaluate.
On the other hand, addition-crosslinking systems require platinum catalysts, which are often considered to have sensitizing properties and moreover incur high raw-material costs. These catalysts are moreover very susceptible to inhibition by chemical compounds which are ubiquitous in the environment, known as catalyst poisons (e.g. amines, thiols). This sometimes considerably restricts the practical use of said systems. Furthermore, there can be a certain excess proportion of the SiH-functional crosslinking agent in addition systems, and this causes post-crosslinking of the vulcanizate. The instability of the SiH function with respect to atmospheric oxygen in the presence of the Pt catalyst leads to conversion to an Si—OH group, which then reacts with further Si—OH groups with elimination of water. The “compression set” resulting from this is a typical disadvantage of addition systems, and can only be mitigated by annealing, which has high energy cost, or by adding specific additives.
These disadvantages are among the reasons for a high level of interest in the use of alternative crosslinking mechanisms, which is a subject of current research. However, processes developed hitherto do not provide any significant advantages, and indeed sometimes result in additional disadvantages. By way of example, the vulcanization of polysiloxanes having a high concentration of vinyl groups, using sulfur or thiols, causes impairment of mechanical properties. Dehydrocondensation between Si—H and Si—OH groups as crosslinking reaction can only be used for the production of thin layers (coatings), since large amounts of hydrogen are produced, and this can foam the material. Radiation-induced crosslinking demands high doses of radiation. The efficiency of the crosslinking reaction under standard conditions is reduced by atmospheric oxygen, and undesired side-reactions occur.
There has long been a need for a crosslinking system for multicomponent silicone rubbers which harden at room temperature to give elastomeric materials without the abovementioned disadvantages. Surprisingly, it has been found that Cu(I)-catalyzed 1,3-dipolar[2+3]cycloaddition (explained in more detail below) between terminal alkynes and azides (hereinafter referred to simply as the “click reaction”) has excellent suitability for this purpose.
The origin of Cu(I)-catalyzed 1,3-dipolar[2+3]cycloaddition between terminal alkynes and azides, generally known today as the “click reaction”, is found in the uncatalyzed, thermal variant of the reaction studied by Huisgen and Szeimies [Huisgen, R.; Szeimies, G.; Moebius, L.; Chem. Ber. 1967, 100, 2494]. This reaction permits the synthesis of 1,4- and 1,5-disubstituted aromatic 1,2,3-triazoles under simple conditions. The catalyzed version of the reaction was discovered in 2002 by Sharpless, who recognized its potential as a highly efficient method of providing linkage between any desired chemical “units” [Lewis, W. G.; Green, L. G.; Grynszpan, F.; Radic, Z.; Carlier, P. R.; Taylor, P.; Finn, M. G.; Sharpless, B. K.; Angew. Chem., Int. Ed. 2002, 41, 2596]. Sharpless utilized the reaction in the context of the concept which he had previously named “click chemistry”, for the synthesis of biologically active substances and of polymers. The catalytic action of Cu+ ions here is based on the formation of a copper acetylide, which is substantially more reactive toward an azide than the original terminal alkyne. The result is a lowering of the activation energy, or an increase in the reaction rate by a factor of 107 at room temperature [Wang, Q.; Chan, T. R.; Hilgraf, R.; Fokin, V. V.; Sharpless, K. B.; Finn, M. G.; J. Am. Chem. Soc. 2003, 125, 3192]. The action of the catalyst is highly specific, requiring retaining the inertness of the two functional groups (azide, alkyne) under the normal physical conditions arising in our environment, and with respect to almost all chemically reactive compounds that occur in these circumstances. The reaction is moreover almost irreversible, contrasting in particular with Diels-Alder cycloadditions. The triazole group formed moreover features high resistance to thermal decomposition and to most reactive chemical compounds, such as oxidants, reducing agents and also acids and alkaline solutions.
These are the reasons for the attractiveness of this type of reaction. Click reactions usually provide almost quantitative conversions, without side-reactions, and are affected very little by external reaction conditions. The click reaction is therefore of great importance for preparative methods in which these features are significant—an example being polymerization reactions.
A brief review of the use of the click reaction in the sector of polymer chemistry and materials science was provided by Lutz, Binder, and Sachsenhofer [Lutz, J.-F.; Angew. Chem. Int. Ed. Engl. 2007, 46, 1018. Binder, W. H.; Sachsenhofer, R.; Macromol. Chem. Rapid. Commun. 2007, 28, 15].
By way of example, the click reaction has been utilized for the production of polytriazoles [Diaz, D. D.; Punna, S.; Holzer, P.; McPherson, A. K.; Sharpless, K. B.; Fokin, V. V.; Finn, M. G.; J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 4392], block-copolymers [Opsteen, J. A.; van Hest, J. C. M.; Chem. Commun. 2005, 57], grafted block-copolymers [Parrish, B.; Breitenkamp, R. B.; Emrick, T.; J. Am. Chem. Soc. 2005, 127, 7404], hydrogels [Ossipov, D. A.; Hilborn, J.; Macromolecules 2006, 39, 1709] and of dendrimers [Wu, P.; Feldman, A. K.; Nugent, A. K.; Hawker, C. J.; Scheel, A.; Voit, B.; Pyun, J. J.; Frechet, M. J.; Sharpless, K. B.; Fokin, V. V.; Angew. Chem., Int. Ed. 2004, 43, 3928. Malkock, M.; Schleicher, K.; Drockenmuller, E.; Hawker, C. J.; Russell, T. P.; Wu, P.; Fokin, V. V.; Macromolecules 2005, 38, 3663].
Another example of the use of click reaction is found in combination with polymeric SiO2 [Rozkiewicz, D. I.; Janczewski, D.; Verboom, W.; Ravoo, B. J.; Reinhoudt, D. N.; Angew. Chem. Int. Ed. 2006, 45, 5292. Rhode, R. D.; Agnew, H. D.; Yeo, W.-S.; Bailey, R. C.; Heath, J. R.; J. Am. Chem. Soc. 2006, 128, 9518. Ranjan, R.; Brittain, W. J.; Polymer Preprints (Am. Chem. Soc., Div. of Polym. Chem.) 2008, 48, 797] and with silicones. In WO2007/132005 A2, for example, the use of silicone-hybrid materials is claimed as emulsifiers for cosmetics.
The use of azido-functional silanes, or organosilicon compounds, for the modification of polymeric materials is likewise known—but not in the context of the “click chemistry” concept. WO 0110914 describes the grafting of azidosilanes onto polyethylene via thermal decomposition of the azide groups. DE 10011644 A1 describes the use of azidosilanes as crosslinking agents in coating materials. The reaction of the azide groups here is brought about either via thermal decomposition or via activation by means of electromagnetic radiation.
Despite these versatile applications, the click reaction has not hitherto been used as crosslinking mechanism for the production of elastomeric silicone plastics.